When a part of the photograh collection goes offline, typically when removing an external HDD, Darktable allows to search the collection and to visualize the thumbnails. Then, when trying to open a photo in the darkroom. the program notifies that the file is not available for editing. That's looks the correct behaviour to me, a great feature to search for archived photographs.

Nevertheless, the lighttable interface still allows to apply styles, history stack copy/paste, and maybe more, to an offline image, and then the thumbnail is lost, replaced by a skull, until the file goes online and reprocesses. In that case it's no longer possible to evaluate the photo visually.

It would be a great idea to deactivate the xmp manipulations of an offline file in the same way the darkroom is, and/or add a visual mark on the thumbnail to indicate when a file is offline.

I too doubted a lot about whether to put it as a bug or as a feature. I made it a bug because although there is no module broken, the global logic of the application is not respected in my point of view: one way of edition is blocked, but other ways are not, and these offline modifications lead to a temporal information loss (in v1.1.4, I didn't test it in v1.2) when trying to apply modifications and recalculate the thumbnail I guess.